West Midlands Police have launched the appeal after returning to the scene of the incident in Perry Barr, which lead to the death of Liam Mooney.
He fell from the moped and suffered serious head injuries which he subsequently died from the next day.
The force said it was still trying to establish exactly what happened, but said the silver car drove off and they need to trace the vehicle and occupants.
The rider of the bike that Liam was on has been spoken to as a witness, as have two people who were on the second moped.
Detective Inspector Stuart Mobberley, said: "Officers will be on Rocky Lane today speaking with drivers and members of the public in the hope that someone might have information about what happened last Monday evening.
"I would encourage the driver of the car that made off to do the right thing and come and speak to us about what happened. It’s important we hear their side of the story.
"Our thoughts remain with Liam’s family at this awful time. They deserve to know what happened to their son that night."
The force has asked anyone who was in the area at the time and saw what happened, or has dash-cam or cycle-cam footage, to get in touch.
A message can be sent via live chat at west-midlands.police.uk between 8am and midnight or call 101 anytime, quoting log number 3771 of 22/03/21.